Isidro Carrascal

Determination of the mechanical properties of normal and calcified human mitral chordae tendineae.

The aim of the present research is to determine the influence of the calcification of human mitral valves on the mechanical properties of their marginal chordae tendineae. The study was performed on marginal chords obtained from thirteen human mitral valves, explanted at surgery, including six non-calcified, four moderately calcified and three strongly calcified valves. The mechanical response of the chords from the non-calcified and moderately calcified valves was determined by means of quasi-static tensile tests (the poor condition of the strongly calcified valves prevented them from being mechanically characterised). The material parameters that were obtained and analysed (the Youngs modulus, the secant modulus, the proportional limit stress, the ultimate strength, the strain at fracture and the density of energy stored up to maximum load) revealed noticeable differences in mechanical behaviour between the two groups of mitral chordae tendineae. Large scatter was obtained in all cases, nevertheless, considering the mean values, it was observed that the normal chords are between three and seven times stiffer or more resistant than the moderately calcified ones. On the contrary, the results obtained for the strain at fracture showed a rather different picture as, in this case, no significant differences were observed between the two families of chords. A scanning electron microscopy study was conducted in order to find out the relevant features of the calcium deposits present in the calcified chordae tendineae. In addition, the general aspects appreciated in the stress vs. strain curves were correlated with the collagen morphological evidences determined microscopically. Finally, the calcium content present in the three groups of chords was quantitatively determined through atomic absorption spectroscopy; then, the relation between the mechanical properties of normal and moderately calcified chords as a function of its calcium content was obtained. This analysis confirmed the existence of a strong correlation between calcium content and stiffness or resistance whereas the influence on the ductility seems to be negligible.

On the application of the theory of critical distances to the structural integrity assessment of stress risers

Purpose – The purpose of this paper is to present and validate a methodology for the structural integrity assessment of components containing a variety of stress risers and subjected to static conditions.Design/methodology/approach – The methodology is based on the use of the apparent fracture toughness prediction provided by the theory of critical distances (in this case, the line method), together with a well‐known, widely‐used engineering tool in structural integrity assessments: failure assessment diagrams. In order to validate the proposed methodology, an experimental programme has been conducted, testing 38 specimens made of aluminium alloy Al7075‐T651, each of them containing a certain stress riser. The comparison between the experimental results and the corresponding predictions provided by the proposed assessment methodology has also allowed the situations for which the theory of critical distances provides accurate predictions to be defined.Findings – The results show that the methodology provid...

Analysis of Notch Effect in Fracture Micromechanisms

This paper presents an analysis of the notch effect in fracture micromechanisms. To this end, experimental results obtained in notched specimens are presented, together with the corresponding stress field at fracture and the SEM fractographies. The specimens comprise three materials (structural steel S275JR, high-strength aluminum alloy Al7075-T651 and Polymethyl methacrylate-PMMA) and notch radii varying from 0 mm (cracks) up to 2.5 mm. The results show how the stress relaxation caused by the notch effect is accompanied by a progressive change in the fracture mechanisms, from basically brittle ones in cracked conditions (for the three materials analyzed) to non-linear mechanisms observed for high notch radii, which explain the increase caused by the notch effect in both the load bearing capacity and the apparent fracture toughness. Also the concept of critical radius, that one below which the notch effect is negligible, is justified by SEM observations.Copyright

Effect of Cutting Method on Fatigue Crack Initiation and Fatigue Life of Structural Steel S355M

This paper analyses the effect of the cutting method on both the fatigue crack initiation and the fatigue life of steel S355M. The research covers three cutting methods (plasma, laser and oxy-fuel) and two specimen geometries: plain specimens with rectangular section and cut edges, and specimens with machined edges and a cut hole in the middle section. All the specimens were conducted to failure in a resonance machine by applying fatigue cycles, the stress ratio (R) being 0.1, and the corresponding S-N curves were obtained for each combination of cutting method and specimen geometry. The crack initiation time was estimated by analyzing the evolution of the resonance frequency on each specimen. The results show a significant influence of the cutting method on both the crack initiation and the fatigue life of this particular steel.Copyright

Orientation of whole bone samples of small rodents matters during bending tests

The influence of the orientation of rat bones on their mechanical response is analyzed in this research. 28 femora obtained from 14 Sprague-Dawley rats were subjected to three-point bending tests, comparing the anteroposterior and posteroanterior orientations. The results show that the whole-bone loading capacity of the femora tested in the posteroanterior orientation clearly exceeds that of the anteroposterior oriented bones. Likewise, the intrinsic (tissue-level) loading capacity of the bones tested in the posteroanterior orientation is manifestly higher than that of the bones tested in the opposite direction. The analysis carried out shows that applying beam theory for symmetric cross-sections leads to underestimating the stress state in the cross-section. In this sense, it is generally recommendable to use the non-symmetric beam theory in order to obtain the normal stresses during bending tests. The geometric, intrinsic and global changes resulting from the orientation of the bones was assessed, finding out that it is the variation in the intrinsic properties which explains the change measured in the whole-bone properties. The experimental scope was increased, including 8 additional femora on which a series of Vickers tests were carried out in the anterior and posterior regions of the cross-section. In all cases the hardness obtained in the anterior region is larger than in the posterior region. This result confirms that the mechanical properties of the bone tissue depend on its position in the cross-section and provides a reliable explanation to understand the response of the bones when subjected to bending tests. These results stress the importance of reporting the orientation of the bones in any scientific paper because, otherwise, it would be impossible to properly assess its impact and relevance.

Effect of Thermal Cutting Methods on the Fatigue Life of High Strength Structural Steel S690Q

This paper analyzes the effect of different cutting methods on the fatigue life of high strength steel S690Q. The research covers three cutting methods (oxy-fuel, plasma and laser) and two specimen geometries: plain specimens with rectangular sections and cut edges, and specimens with machined edges and a cut hole in the middle section. All the specimens were conducted to failure by applying fatigue cycles, the stress ratio (R) being 0.1, and the corresponding S-N curves were obtained for each combination of cutting method and specimen geometry. Measurements of roughness and hardness have been performed in order to explain the influence of the cutting method on the fatigue life of this particular steel. Fatigue results have been compared with the predictions provided by current fatigue standards, analyzing the possibility of extrapolating their S-N curves, focused on oxy-fuel cuts, to plasma and laser cuts.Copyright

Quasidistributed fiber sensor for precast concrete structures monitoring

The prestressing force is transferred to the concrete during the slack period of precast concrete structures fabrication. To contribute to a better understanding of the armor compliant behaviors during the mentioned process, a custom quasidistributed Bragg Grating optical fiber sensor system is designed, fabricated and embedded into a prestressed concrete prismatic beam. The experimental works, the results, their discussion and finally the obtained conclusions are presented in this paper.

Validation Through Finite Element Simulation of the Behaviour of a Polyurethane Shock Absorber Under In-Service and Extreme Conditions

The safety rules for the construction and installation of lifts, currently in force in Europe, include several requirements concerning the behaviour of the shock absorbers when stopping an elevator. In this paper, a finite element model simulating the behaviour of a cellular polyurethane shock absorber has been developed. The material mechanical behaviour was simulated by means of an elastomeric foam theoretical model, previously calibrated in a former paper. Several in-service and extreme condition scenarios have been analysed with this numerical model, thus verifying the fulfilment of the requirements of the standard.

Structural Integrity Assessment of Notched Components

This paper analyzes the notch effect and presents a methodology, based on failure assessment diagrams and the notch analysis approaches based on the theory of critical distances, for the structural integrity assessment of notched components, which allows more accurate structural analyses to be made. The methodology is applied to a set of tests performed on PMMA single edge notched bending (senb) specimens, providing better results than those obtained when the analysis is performed considering that notches behave as cracks.Copyright

Characterization of Mechanical Properties of a Shock Absorber Polyurethane Foam for Elevators. Numerical Fitting of Mechanical Behavior Models for Hyperelastic and Elastomeric Foam Materials

The elastic cellular polyurethane elastomer is widely used to manufacture shock absorbers for elevators, due to its excellent con- ditions for absorption of energy and vibration damping. In this paper, a complete mechanical characterization of this material was performed including the uniaxial compressive test, the planar test, and the volumetric and the simple shear test. From the experimental results, several models of behavior for hyperelastic and elastomeric foam materials have been analyzed by fitting their corresponding material parameters.The scope of this work includes the Ogden model, the Van der Waals model, and polynomial and elastomeric foam forms.